Numerous studies demonstrate that pupil diameter reacts not only to the surrounding luminosity but also to emotional arousal and cognitive workload. Generally, any type of activation leads to pupil dilation. Pupil reactions to the prolonged workload is less well studied. In our study a subject had to solve arithmetic tasks and choose the right answers for 60 or 90 minutes. We were performing pupillometry via eye tracking. 16 subjects (age 18–28) took part in the study. Pupil diameter was a marker of a subtask: mental calculation or selection of a right answer. Searching for an answer caused an overall activation and significant pupil dilation. The scope and dynamic of this reaction have shown individual variability. We have not found any significant correlation between pupil diameter and time spent on each task. Moreover, in three subjects we have detected a phenomenon of hippus, slow pupil oscillations in constant light conditions which may accompany mental work or fatigue. We suppose that pupil diameter can signal a rapid change in arousal or activation but is not a reliable marker of a current task difficulty, especially when performing a relatively monotonous task.